The long-term objective of this project is to extend the application of stem cell transplants for use in patients with nonmalignant diseases by developing less toxic conditioning regimens that permit engraftment and prevent graft-versus-host disease. Included among these patients are those with hypocellular marrows who require immunosuppression only (e.g. aplastic anemia), those with genetic diseases in which 20-30% donor chimerism would relieve disease symptoms, and prospective solid organ transplant recipients who would become tolerized to their grafted organ. To address this goal, nonmyeloblative conditioning programs for hematopoietic allografts witch minimal graft-versus-host disease will be developed in a well-established preclinical model of random-bred dogs. The source of marrow repopulating cells used in these studies will be cytokine mobilized peripheral blood stem cells (PBSC) which enable earlier engraftment than does marrow. Specifically, we propose to explore regimens that will include monoclonal antibodies (MAbs) to lymphocyte surface determinants. Depending on the histocompatibility setting studied (DLA-identical vs. DLA-haploidentical littermates), MAbs to the TCR/CD3 complex, CD4, CD8, or certain adhesion molecules, e.g. CD44, may be used alone or in combination. The role of postgrafting immunosuppression, used to prevent GVHD, on engraftment will be defined. To address the issue of stem cell homing, as would be encountered in patients with cellular marrow, e.g. sickle cell disease, we shall determine how much marrow ablation in addition to immunosuppression is required for engraftment to be assured. This may be accomplished by very low-dose TBI, local marrow radiation, or the use of an MAb coupled to a short-lived alpha-emitting radionuclide, astatine-211. Finally, in DLA- haploidentical grafts, we will use a transient graft-versus-host effect to help with engraftment by transplanting "lineage-depleted" PBSC along with in vitro expanded donor-anti host CTL that have been transduced with a herpes simplex virus thymidine kinase gene. Consequently, we shall exploit the CTL's acquired susceptibility to ganciclovir and eliminate the CTL after PBSC engraftment has occurred but before CVHD develops.